LT8708-1
16
Rev 0
For more information
As another example, the master LT8708’s current regula-
tion pins (IMON_INP, IMON_INN, IMON_OP, IMON_ON)
monitor and limit the per-phase V
IN
current and V
OUT
cur-
rent. The LT8708-1 complies with these limits by regu-
lating the I
OUT(SLAVE)
proportionally. The slave’s IMON
pins are typically used differently than on the LT8708.
: Configuration section and the Current
Monitoring and Limiting section for more information.
Finally, the VINHIMON and VOUTLOMON pins can be used
to set up V
IN
overvoltage and V
OUT
undervoltage lockouts
on both the LT8708 and the LT8708-1. Typically, however,
divider networks are only necessary on the master LT8708
since activation of the VINHIMON or VOUTLOMON com-
parator, on any phase, is communicated to all phases
through the shared
RVSOFF
pin connection. Utilizing the
VINHIMON and VOUTLOMON pins on additional phases
offers redundancy for those functions. See the VINHIMON,
section for more details.
Adding Phases: The Slave LT8708-1
Further information about the LT8708-1 in Figure 2 is
as follows:
• The ICP and ICN signals connect between the LT8708
and all the LT8708-1s. They are used to deliver the
positive and negative I
OUT(MASTER)
information from
the LT8708 to the LT8708-1(s), and hence set the
average regulated I
OUT(SLAVE)
.
• Typically, the LT8708-1 operates by regulating its
I
OUT(SLAVE)
to a proportion of the I
OUT(MASTER)
. The
LT8708-1’s IMON_OP pin regulates to 1.209V as a
part of this regulation. This IMON_OP function differs
from the LT8708 in that the LT8708-1’s IMON_OP
is not part of the positive I
OUT(SLAVE)
monitor func-
tion. Always connect a 17.4k resistor in parallel with
a compensation network from this pin to ground on
the LT8708-1.
• The
IMON_ON pin is used to monitor the negative
I
OUT(SLAVE)
. The current limiting function of this pin
on LT8708-1 is disabled and is instead controlled by
the master LT8708.
• The
LT8708 and LT8708-1 employ different soft-start
mechanisms, and the SS pins ramp up differently.
See the Start-Up: Soft-Start of Switching Regulator
section for more details.
Apart from the information explained above from Figure 2,
a few other pins also need to be considered when config-
uring a multiphase system. A summary of pins and their
recommended usage is provided in Table 2.
Table 2. Summary of Pin Connections
SHORT PINS
BETWEEN LT8708
AND LT8708-1
PIN NAME(S)
NOTES
YES
SWEN,
RVSOFF
Open-drain communication
between all phases. Keeps LT8708/
LT8708-1(s) in same states.
ICP, ICN
Send LT8708’s I
OUT(SLAVE)
information to LT8708-1.
MAYBE
MODE, DIR
Typically, pins are driven to the
same states as LT8708.
NO
FBOUT,
IMON_INN
Disable these error amps on
LT8708-1(s), or set to same or
higher limits than LT8708 if used
for secondary limits.
IMON_INP
If using RHCM, connect a
17.4k resistor and parallel
filter capacitor from this pin to
ground. Otherwise, disable this
error amp or set to same or
higher limit than LT8708 if used
for secondary I
IN(SLAVE)
limit.
VINHIMON,
VOUTLOMON
Comparator states are shared
between LT8708/ LT8708-1(s)
through
RVSOFF
pins. Pins on
LT8708-1 can be disabled or
used as redundant detectors.
IMON_OP
On LT8708-1, connect a 17.4k
resistor and a compensation
network from this pin to ground.
IMON_ON
Limiting function is disabled
on LT8708-1. If using FHCM,
connect a 17.4k resistor in
parallel with a filter capacitor
from this pin to ground to
properly detect light load.
SS, RT
Use same value capacitor,
resistor as LT8708.
LDO33,
INTV
CC
,
GATEV
CC
Typically would have same
capacitors as LT8708.
OPERATION
